The present invention relates to a method and a device for modulating optical signals based on modulation of the absorption and of the bending losses in bend, quantum well semiconductor waveguide sections. The complex refractive index of the optical active semiconducting components of the waveguide section is modulated through the Quantum Confined Stark Effect (QCSE), by applying a variable electric or electromagnetic (EM) field. The modulation results in a modulation of the effective refractive index contrast and the absorption coefficient for the waveguide at the frequency of theoretical signal.
In optical communication it is often of interest to obtain a high bit rate in the optical signals, the improvement of the present standards of 10 Gbit/s being restrained by the modulation speed of optical modulators. Typically, two classes of optical modulators are used, interferometric devices such as Mach-Zehnder type modulators and Electro-Absorption Modulators (EAMs).
Mach-Zehnder modulators utilise optic active materials to control a phase shift between two arms in an interferometer whereby the resulting signal may be modulated. Mach-Zehnder modulators presently provide modulation speeds up to 40 Gbit/s, however, 100 Gbit/s have been reported. It is a disadvantage of Mach-Zehnder modulators that they are typically large, expensive, and require a large voltage amplitude to produce the required phase shift.
In EAMs, a modulated absorption coefficient is induced in active semiconductor materials using a modulated electric field, i.e. utilising QCSE. There are two characteristic energy regimes for a semiconductor material, being denoted as below bandgap and above bandgap, where the absorption coefficient (proportional to the imaginary part of the refractive index) of the semiconductor material is zero or non-zero, respectively. This is shown schematically in FIG. 1 where the curves show no absorption at low energies/frequencies below bandgap and high absorption at high energies/frequencies above bandgap. The boundary between these two regimes, i.e. the bandgap region where the curves rise steeply, can be shifted due to the Quantum Confined Stark Effect (QCSE) when the material comprises a Quantum Well semiconductor structure. This is also shown in FIG. 1, where the absorption (i.e. the imaginary part of refractive index) are shifted to lower energies/lower frequencies when a reverse bias is applied. The QCSE in bulk structures is denoted the Franz-Keldysh Effect (FKE). The QCSE is observed when reverse biasing the semiconductor structure. The amount of absorption near the bandgap is thereby increased for increasing reverse bias. Thereby, the optical absorption may be modulated between a low and a high value for light in a narrow energy/frequency region as indicated by the shadowed region 2 in FIG. 1. The change in the absorption due to the QCSE or FKE is the mechanism used in EAMs. Presently, EAMs can provide modulations speeds up to 40 Gbit/s.
In an article by Veldhuis et al, Optics Communications, 168 (1999) 481, an optic intensity modulator based on a bent channel waveguide is disclosed. The bent channel waveguide has a fixed bending radius . When the lateral refractive index contrast between the core and the cladding material is high enough, all the light in the waveguide will be guided. If the lateral refractive index contrast is lowered sufficiently, part of the light will be radiated out of the waveguide, the exact fraction depending on the value of the contrast. By adjusting the contrast, the precise transmitted power may be controlled. FIG. 6 summarises the length and changes nact in the refractive index of the core, assuming constant cladding index, required to achieve a 30 dB extinction. Veldhuis et al proposes the use of thermo-optic or electro-optic actuation for controlling the refractive index in thermo-optic or electro-optic polymers applied in NxM matrix switches to decrease cross-talk and increase compactness.
It is an object of the present invention to provide a device and a method for modulating electromagnetic (EM) radiation, which provide an enhanced extinction ratio and faster operation than conventional modulators.
It is another object of the present invention to provide a device and a method for modulating EM radiation, which provides more compact modulators than conventional modulators.
It is a further object of the present invention to provide a device and a method for modulating EM radiation, which requires smaller voltage swings than conventional modulators.
The response of an optical active semiconductor material to an EM field (light) is governed by the complex refractive index of the semiconductor material, generally denoted as n=Re(n)+i Im(n). The imaginary part Im(n) determines the amount of light, which will be absorbed in the semiconductor material, while the real part Re(n) of the refractive index determines the speed of light in the medium. The refractive index is a function of frequency and the amount of absorption hence depends on the frequency (or wavelength) of the light.
The lateral confinement of light in typical waveguides is based on total internal reflection. Total internal reflection is the reflection of EM radiation from the interface of a medium with larger index of refraction n1 with a medium of smaller index of refraction n2 less than n1 when making an angle of incidence T! sin 1       n    2        n    1  
to normal. Thus, the lateral confinement of light depends upon the index contrast between the waveguide core and the surrounding material as well as upon the angle of incidence of light on the boundaries between the waveguide core and the surrounding material. Hence a change in the index contrast may, depending on the angle of incidence, introduce losses du to lack of total internal reflection. Also, varying the direction of the lateral confinement parts or side walls will change the angle of incidence and may, depending on the index contrast, introduce large losses due to lack of total internal reflection. Variations in the direction of the lateral confinement parts may be a bent waveguide if both sides of the lateral confinement vary identically. It may also be a variation of only one of the sides such as a narrowing of the waveguide. Alternatively, the width of the waveguide may vary in that both sides performs repeated change of directions, such as a wobbling. All these different scenarios will introduce losses since they change the angle of incidence on at least one side of the lateral confinement boundary of the waveguide, the collective term bending losses will be used for simplicity.
The present invention provides an optical intensity modulation by introducing a modulated loss governed by a modulation of the real part of the refractive index. The invention may be implemented as an optical modulator based on these modulated losses alone, or may be used to improve the performance of existing optical modulators by introducing an extra loss for improving the extinction ratio. A modulation of the real part of the complex refractive index can be accomplished in different ways, however, in order to obtain modulation speeds fast enough for industrial application in optical communication and related fields, the working principle and material composition of devices must be carefully considered. The present invention will provide more compact optical intensity modulators with improved performance (extinction) and speed.
It is known from prior art electro-absorption modulators to use QCSE to obtain a modulation in the absorption (or equivalently the imaginary part of the refractive index). The modification of the refractive index, shifted due to the QCSE in case of a quantum well semiconductor structure comprising an optical active semiconducting material core, is not only restricted to the imaginary part of the refractive index. Also the real part of the refractive index will be modified. The change of the real part can be calculated from the changes in the imaginary part of the refractive index by the Kramers-Kronig transformation. In general, QCSE provides a very fast and precise variation of the real part of the refractive index, however, the QCSE can only change the real part a small amount (the effective refractive index can be changed on the order of a few 10xe2x88x923). Thus, the effective refractive index contrast as well as the bandgap frequency of the bandgap is modulated by the electrical field applied to induce QCSE, also referred to as the voltage swing or V.
The complex refractive index modulations may also be induced by photo generated charge carriers. In this case, the modulation of the complex refractive index is a result of different effects following from the absorption of an EM field such as an intensity modulated optical control signal. The control signal should have a frequency above the bandgap of the optical active semiconducting material in order to be absorbed and generate free charge carriers in the material. If the optical active semiconducting material core is electrically biased, the photo-induced free charge carriers will screen the bias field and thereby modulate the bandgap energy according to the QCSE, hence the term optically induced QCSE. Furthermore, the photo-induced free charge carriers affect the complex refractive index of the material, resulting in the desired index modulations.
The present invention fulfil the objects given above by, in a first aspect, providing an optical modulator for modulating electromagnetic (EM) radiation having a first frequency "THgr"1, said optical modulator comprising a first waveguide section for guiding the EM radiation, said waveguide section comprising an elongated core region with complex refractive index ncore having side walls to a surrounding region with complex refractive index nsurr, the difference between the real part of ncore and the real part of nsurr defining a refractive index contrast n=Re(ncore)xe2x88x92Re(nsurr) and at least one of the side walls of the core region being in the longitudinal direction of the core region, and comprising means for applying a modulated first and second electric or EM field E1 and E2 to the core region, wherein the core region comprises an optical active semiconducting material having a predetermined material composition and having an energy bandgap, said energy bandgap being positioned at a first bandgap frequency "THgr"bandgap E1 in response to the application of the first field and being positioned at a second bandgap frequency "THgr"bandgap E2 in response to the application of the second field, ncore depending upon the energy bandgap so that the material composition provides, for EM radiation of the first frequency, a first complex refractive index ncore E1 in response to the application of the first field and a second complex refractive index ncore E2 in response to the application of the second field, and wherein the predetermined material composition and the first frequency are chosen so that a difference in the index contrasts nE1=Re(ncore E1)xe2x88x92Re(n) surr) and nE2 =Re(ncore E2) xe2x88x92Re(nsurr) results in bending losses for EM radiation of the first frequency guided in the waveguide.
Thus, according to the first aspect, the index contrast for a waveguide section having an optical active semiconducting material core may be modulated electronically or optically. If at least one side of the lateral confinement boundary of the waveguide is bend, the modulation of the index contrast will result in losses due to lack of total internal reflection. Naturally, the bend must be in the longitudinal direction of the core region so as to intercept the straight propagation of light in the waveguide. These losses may efficiently improve the performance of existing absorption-modulators as described in the following.
As mentioned previously, QCSE can only change the real part of the refractive index of an optically active semiconductor material a small amount, typically on the order of a few 10xe2x88x923, similarly, the shift in the index contrast due to QCSE induced by the applied fields will be correspondingly small resulting in a small extinction due to bending loss modulation. Thus, in order to induce high extinction ratios in the modulated bending losses, the waveguide index contrast should be small at least in directions in the plane of the bend (transverse directions). Therefore, the index contrast between the core region and the surrounding regions in the lateral direction is preferably equal to or smaller than a few 10xe2x88x922 such as equal to or smaller than a few 10xe2x88x923.
By modulating the bandgap frequency of the bandgap of an optically active semiconductor material, the absorption coefficient will also be modulated whereby some absorption will occur in the material. However, the absorption of the EM radiation generates free charge carriers in the active material. These free charge carriers may screen the applied electrical field and are a part of the circuit performing the modulation, hence, the transporting of these free charge carriers will effectively limit the modulation speed. At high light intensities, the generated free charge carriers may ultimately saturate the.circuit performing the modulation.
However, since the radiation in the waveguide will experience large bending losses as well, the amount of absorbed photons will be reduced, thereby also reducing the generation of free charge carriers. Thus, by combining the absorption modulation scheme with bending losses, the combined extinction from absorption and bending losses will increase the obtainable modulation speed and extinction. The combined extinction from absorption and bending losses will enhance the extinction ratio of the modulator for a given amplitude in the applied fields, voltage swing if an electrical signal is applied and modulation depth if an EM signal is applied.
As mentioned in the above, in order to induce high extinction ratios in the modulated bending losses, the waveguide index contrast should be small at least at positions where the bending losses is supposed to take place. Therefore, the waveguide type is preferably a weakly index guided waveguide such as a ridge waveguide or a Buried Heterostructure (BH) waveguide. The waveguide typically comprises a number of different material layers deposited on a substrate, the different layers forming a structure which define the core region in one transverse (typically vertical) direction. At least one of the material layers forming the core region is an optically active semiconductor material meaning that it has an energy bandgap above which the material can absorb photons.
The means for applying the fields are preferably one or more electrical contacts for forming an electric field, the contacts being formed by one or more electrically conducting material layers deposited on the waveguide structure. Forming and contacting such contacts are well known techniques within the field of planar waveguides.
Alternatively, the applied field is an EM field. In this case, the means for applying the first and second fields comprises one or more optical input ports for receiving an EM signal of a second frequency and means for guiding said signals to the core region. The optical active semiconducting material and the second frequency of the EM radiation should be chosen so that the radiation is absorbed.
In one preferred type of waveguides, the horizontal transverse boundaries are defined by an electric field applied over only part of the active layer so as to induce an index contrast in horizontal transverse direction of the active layer. Alternatively, the core region is defined in the horizontal transverse by a material region having a slightly different refractive index. Optionally, the lateral confinement is provided by a combination of these effects.
As can be seen from FIG. 1, the bandgap is not defined by a single frequency, rather there is an energy boundary region wherein the absorption increases for increasing energies. By choosing a given absorption within this boundary region, it is possible to define corresponding bandgap frequency, "THgr"bandgap, at which the bandgap starts, and above and below which one refers to above bandgap or below bandgap. It is important to stress that the bandgap frequency can be chosen anywhere in the boundary region, and since one often works in the boundary region, a frequency being higher than the bandgap frequency simply means that light having this frequency experiences a higher absorption than light having a frequency lower than the bandgap frequency. Hence a given frequency in the boundary of FIG. 1 may be above bandgap in a first situation (where it is compared to an even lower frequency experiencing a smaller absorption) whereas it will be below bandgap in another case (compared to a slightly higher frequency experiencing a higher absorption). Therefore, it is generally not possible to assign a specific bandgap frequency to one of the curves in FIG. 1, as it depends on the specific situation.
Preferably, the bending losses are introduced by applying a bent waveguide section. Alternatively, the waveguide is designed so that at least one of the side walls is bent so as to vary the width of the waveguide. Such design will also introduce bending losses due to the many small bends in the sections. Also, different types of bending losses may be combined.
The combination of absorption and bending losses enhance the extinction ratio for a given amplitude of the applied field. The effect that the bending losses reduce the amount of free charge carriers leads to a number of advantages, most important an increased modulation speed and reduced tendency for the free charge carriers to saturate the modulation circuit. These effects make the modulator more efficient which allows a decrease in the size of the modulator while keeping the efficiency (extinction ratio) constant. The increased efficiency of the extinction also allows for smaller and thereby faster electrodes for applying the field.
Depending on the exact purpose and design of the waveguide, the ratio between the contributions from the two means of extinction, absorption and bending losses, may be varied. If the optical signal to be modulated, i.e. the EM radiation of the first frequency, has a frequency below the bandgap frequency of the predetermined material composition (with the applied field), only modulated bending losses will be introduced. If the optical signal to be modulated has a frequency above the bandgap frequency, the modulation in the imaginary part of the refractive index will introduce modulated absorption losses whereas the modulation in the real part of the refractive index will introduce modulated bending losses. Thus, by controlling the material composition and the first frequency, the extinction can be precisely controlled.
Thus, in a first preferred embodiment, the predetermined material composition of the optical active semiconductor material is preferably adjusted so that, for EM radiation of the first frequency, the first complex refractive index, ncore E1 and the second complex refractive index, ncore E2 fulfil the relations:
I. Re(ncore E1) greater than Re(ncore E2) giving a first refractive index contrast nE1 if the first field is applied and a second refractive index contrast nE2 if the second field is applied, the first refractive index contrast being larger than the second refractive index contrast, nE1 greater than nE2,
II. Im(ncore E1) less than Im(ncore E2), giving a first bandgap frequency larger than the first frequency, "THgr"bandgap E1 greater than "THgr"1, in response to the application of the first field and a second bandgap frequency smaller than the first frequency, "THgr"bandgap E2 less than "THgr"1, in response to the application of the second field.
In this preferred embodiment, when the first field is applied, the index contrast is high and the first frequency is below bandgap resulting in an efficient, low loss guiding through the bent section and a low absorption. Hence, when the first field is applied, both means of extinction work to give a high transmission through the waveguide section. When the second field is applied, the index contrast is small resulting in large bending losses, and the first frequency is above bandgap resulting in a high absorption. Hence, when the second field is applied, both means of extinction work to give a large extinction in the waveguide section.
This situation is illustrated in FIG. 2 for the case where the applied field is an electric field. In FIG. 2, wherein the predetermined material composition of the optical active semiconductor material is adjusted so as for the first frequency to lie in the shaded region. In FIG. 2, the real and imaginary part of the refractive index of the semiconductor material are given as a function of energy for two different applied fields such as no bias and negative bias corresponding to the first and second field respectively. It can be seen that when the second field is applied, the QCSE shifts the values of Re(ncore) and Im(ncore) to values resulting in increased bending losses and absorption. Due to the bending losses, the extinction ratio is enhanced and the amount of photo-generated free charge carriers is reduced, both effects contributing to a more efficient modulation allowing for a reduction in size and voltage swing compared to existing EAMs.
In a second preferred embodiment, the predetermined material composition of the optical active semiconductor material is preferably adjusted so that, for EM radiation of the first frequency, the first complex refractive index, ncore E1 and the second complex refractive index, ncore E2 fulfil the relations:
I. Re(ncore E1) less than Re(ncore E2) giving a first refractive index contrast nE1 if the first field is applied and a second refractive index contrast nE2 if the second field is applied, the first refractive index contrast being smaller than the second refractive index contrast, nE1 less than nE2,
II. Im(ncore E1)|Im(ncore E2) resulting in a bandgap frequency larger than the first frequency if either of the first or second field is applied, "THgr"bandgap E1 greater than "THgr"1 and "THgr"bandgap E2 greater than "THgr"1.
In the second preferred embodiment, the bending losses give the major contribution to the extinction. When the first field is applied, the index contrast is small resulting in large bending losses, and the first frequency is below bandgap resulting in a low absorption. Hence, the extinction results primarily from the bending losses. When the second field is applied, the index contrast is high resulting in an efficient, low loss guiding through the bent section, and the first frequency is below bandgap resulting in a low absorption. Hence, when the first field is applied, only the bending losses work to give a large extinction, whereas when the second field is applied both effects allows for an efficient low-loss guiding in the waveguide section.
This situation is illustrated in FIG. 3, where the applied field is an electric field. In FIG. 3, the predetermined material composition of the optical active semiconductor material is adjusted so as for the first frequency to lie in the shaded region. It can be seen that when the second field is applied, the QCSE shifts the values of Re(ncore) to values resulting in increased bending losses while Im(ncore) is very small and do not change significantly. Typically, Im(ncore,E1) is slightly smaller than Im(ncore,E2) meaning that the absorption is larger when the bending loss is low. Therefore, the predetermined material composition of the optical active semiconductor material is preferably adjusted so as for the absorption to be very low when the second field is applied. Therefore, no photo carriers are generated which could otherwise cause modulation speed limitations due to transport times.
When an optical mode propagates in a bent waveguide section, it is shifted toward the outer perimeter of the bend. Therefore, coupling of different waveguide sections is also a source of losses, coupling losses. It is known, e.g. from Veldhuis et al, that these coupling losses depend on the index contrast of the coupled waveguide sections.
Thus, in a third preferred embodiment the optical modulator further comprises a second waveguide section similar to the first waveguide section and positioned in extension of the first waveguide section, said second waveguide section having a coupling to the first waveguide section which is adapted to introduce coupling losses for radiation in the optical modulator, said coupling losses depending on the refractive index contrast in parts of the sections close to the coupling.
Hence, when modulating the refractive index to modulate the bending losses, the modulator may be designed to benefit from the coupling losses"" dependence upon the index contrast. Therefore, the means for applying the first and the second field preferably comprises means for applying the first and the second fields to core regions close to the coupling in the first and/or second waveguide section, so as to modulate the refractive index contrast in these regions.
For all embodiments of the modulator according to the first aspect of the invention, the material composition of the modulator is very important for obtaining the required relations between the complex refractive indices at different applied electric fields. A variety of waveguide designs are applicable, and the design parameters including the material composition may be very precisely determined using existing semiconductor processing technologies. Preferably, the core and/or the surrounding regions are at least substantially formed by one or more of the materials selected from the group consisting of III-V or II-VI semiconductor materials. The II-V material could typically be InP, GaAs, AlGaAs, InGaAsP, whereas a typical II-VI material could be ZnSe.
Depending on the design, it may be necessary to dope one or more material layers, hence the core and/or the cladding region may be doped with one or more of the materials selected from the group consisting of Be, Zn, Mg, Si, C and S.
An optical modulator according to the present invention may advantageously be applied for modulating light signals in order to encode information into the signals. Hence, the means for applying the first and the second electric field preferably comprises one or more electrical contacts for receiving an electric signal and generating the first and second electrical field in response to the received electric signal. The modulator may further comprise ultra fast receivers and amplifiers for receiving the signal. Typically, the received signal will be a binary signal and the means for generating the first and second electrical field is preferably adapted to generate the first field corresponding to xe2x80x9c0xe2x80x9d and the second field corresponding to xe2x80x9c1xe2x80x9d, or vice versa.
The modulator according to the present invention will preferably be used in optical communication. Hence the material compositions are preferably optimised for providing optimum modulation for light having a wavelength in the region from 500 nm to 2000 nm. Preferably, the modulator is optimised for light having a wavelength in the region 750 nm to 900 nm or 1300 nm to 1650 nm, preferably within smaller regions centered at 850 nm, 1350 nm or 1550 nm.
In the case where the applied field is an electric field, the first applied electric field is preferably at least substantially zero and the second applied electric field is negative. This is because the QCSE typically gives the largest shift for negative bias thereby requiring a smaller voltage swing.
According to a second aspect, the present invention provides a method for modulating EM radiation using the optical modulator according to the first aspect.